add machine hook to handle calls to 'extra' function values

src/librustc_mir/const_eval.rs

@@ -371,6 +371,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         }))
     }
 
+    fn call_extra_fn(
+        _ecx: &mut InterpretCx<'mir, 'tcx, Self>,
+        fn_val: !,
+        _args: &[OpTy<'tcx>],
+        _dest: Option<PlaceTy<'tcx>>,
+        _ret: Option<mir::BasicBlock>,
+    ) -> InterpResult<'tcx> {
+        match fn_val {}
+    }
+
     fn call_intrinsic(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
         instance: ty::Instance<'tcx>,

src/librustc_mir/interpret/machine.rs

@@ -124,6 +124,16 @@ pub trait Machine<'mir, 'tcx>: Sized {
         ret: Option<mir::BasicBlock>,
     ) -> InterpResult<'tcx, Option<&'mir mir::Body<'tcx>>>;
 
+    /// Execute `fn_val`.  it is the hook's responsibility to advance the instruction
+    /// pointer as appropriate.
+    fn call_extra_fn(
+        ecx: &mut InterpretCx<'mir, 'tcx, Self>,
+        fn_val: Self::ExtraFnVal,
+        args: &[OpTy<'tcx, Self::PointerTag>],
+        dest: Option<PlaceTy<'tcx, Self::PointerTag>>,
+        ret: Option<mir::BasicBlock>,
+    ) -> InterpResult<'tcx>;
+
     /// Directly process an intrinsic without pushing a stack frame.
     /// If this returns successfully, the engine will take care of jumping to the next block.
     fn call_intrinsic(

src/librustc_mir/interpret/terminator.rs

@@ -237,7 +237,12 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx> {
         trace!("eval_fn_call: {:#?}", fn_val);
 
-        let instance = fn_val.as_instance()?;
+        let instance = match fn_val {
+            FnVal::Instance(instance) => instance,
+            FnVal::Other(extra) => {
+                return M::call_extra_fn(self, extra, args, dest, ret);
+            }
+        };
 
         match instance.def {
             ty::InstanceDef::Intrinsic(..) => {